The present invention relates to a channel detecting method for digital broadcast, which performs the detection of a broadcast channel at high speed, and a receiver using the channel detecting method, such as a digital terrestrial receiver.
FIGS. 2(a) and 2(b) are diagrams showing a conventional method of demodulating a digital broadcast. The same figure (a) is a diagram showing a schematic process of a broadcast wave, and the same figure (b) is a frame configuration diagram of a baseband signal (called broadcast wave signal).
A conventional process of a broadcast wave UHF is performed after the broadcast wave UHF containing a channel number and a physical frequency transmitted from a broadcast station has been subjected to reception processing. The broadcast wave UHF subjected to the reception processing is selected by tuner processing and tuned to a predetermined frequency, after which it is outputted as a baseband signal S11b. The baseband signal S11b is subjected to demodulating processing and thereby outputted as a transport stream (hereinafter called “TS”) signal Sts.
The baseband signal S11b comprises one frame FM with a plurality of transmission symbols SBs as a unit. Each transmission symbol SB is constituted of a guard interval GI and an effective symbol S.
In the conventional digital broadcast demodulating method, when distortion and a multipath exist in a transmission line, the orthogonality of a received signal is damaged and disturbed and hence inter carrier interference (hereinafter called “ICI”) occurs in a demodulated signal, thus degrading an error rate. In order to solve it, ineffective guard intervals GI for ICI absorption are provided as buffer data portions.
As examples of such a conventional digital broadcast channel detecting method and a receiver using the same, there have been proposed ones described in, for example, patent documents (Japanese Unexamined Patent Publication Nos. 2004-179928 and 2005-333190) or the like.
The patent document 1 has described a channel detecting method for causing a digital broadcast channel to be reliably stored in a channel list where a reception environment is unstable, and a receiver using the same. The patent document 2 has described a channel detecting method for acquiring each broadcast parameter contained in a network information table (hereinafter called “NIT”) and performing, by some channels alone, a process for determining whether it corresponds to a targeted broadcast wave, and a receiver using the same.
The digital broadcast receiving method and its receiver described in each of the patent documents 1 and 2 are however accompanied by a drawback that when they are applied to a portable terminal that frequently changes in broadcast area, such as a cellular phone, a car navigation system or the like on the precondition that the receiver is not moved from its installation location, a broadcast channel may be misdetected. In order to solve such a drawback, there has been proposed a digital terrestrial broadcasting receiver mountable onto portable terminals such as a cellular phone that frequently changes in broadcast area such as shown in FIG. 3, a car navigation system and the like.
FIG. 3 is a schematic block diagram showing the conventional digital terrestrial broadcasting receiver.
The digital terrestrial broadcasting receiver has a reception section 11 which receives a broadcast wave UHF and outputs a baseband signal S11b, and an orthogonal frequency division multiplexing (hereinafter called “OFDM”) demodulation unit 12 which demodulates the baseband signal S11b and outputs the demodulated baseband signal S11b as a TS signal Sts. A decoder 13, which decodes the TS signal Sts thereby to output output data Dout therefrom and outputs a decode signal S13, is connected to the output side of the OFDM demodulation section 12. Further, a CPU 14, which outputs control signals S14-1, S14-2 and S14-3, is connected to the decoder 13.
The reception section 11 comprises an antenna 11a for receiving the broadcast wave UHF, and a tuner 11b which selects a channel in a predetermined frequency band with respect to the broadcast wave UHF and tunes in thereto, and outputs a baseband signal S11b. The OFDM demodulation section 12 comprises a synchronous establishment unit 12a which performs synchronous establishment on the baseband signal S11b thereby to output a synchronous playback or reproduction signal S12a, a demodulation unit 12b which performs fast Fourier transform on the synchronous recovery signal S12a thereby to output a demodulated signal S12b, and an error correction unit 12c which corrects an error of the demodulated signal S12b and effects deinterleave processing thereon thereby to output a TS signal Sts.
The operation of the digital terrestrial broadcasting receiver shown in FIG. 3 will next be explained.
The antenna 11a first receives a broadcast wave UHF therein and inputs it to the tuner 11b. The tuner 11b causes the broadcast wave UHF to tune to a predetermined frequency, removes an unnecessary frequency band by an unillustrated bandpass filter and outputs the broadcast wave UHF to the OFDM demodulation section 12 as a baseband signal S11b. 
The baseband signal S11b is first synchronously established by a synchronous establishment unit 12a and outputted as a synchronous recovery signal S12a. The synchronous recovery signal S12a is subjected to demodulating processing at the demodulation unit 12b and outputted as a demodulated signal S12b. The demodulated signal S12b is subjected to an error correction and deinterleave processing at the error correction unit 12c and outputted to the decoder 13 as a TS signal Sts.
The TS signal Sts is separated into multiplexed various information at the decoder 13 and outputted to a speaker, an earphone, a display device and the like unillustrated in the drawing as output data Dout. An image display and voice reproduction are performed based on the output data Dout. Further, the decoder 13 outputs a decode signal S13 corresponding to information about a decode situation to the CPU 14. The CPU 14 outputs a tuning control signal S14-1 to the tuner 11b, outputs a control signal S14-2 to the OFDM demodulation section 12 and outputs a control signal 314-3 to the decoder 13.
Each broadcast parameter for associating a remote control channel number and a physical frequency with each other can be obtained at the decoder 13. In order to obtain the broadcast parameter, the predetermined broadcast wave UPH applied to the tuner 11b is subjected to the demodulating processing at the OFDM demodulation section 12 and outputted as a TS signal Sts. And the corresponding broadcast parameter is extracted from the TS signal Sts at the decoder 13. Upon a broadcast wave scan, the retrieval of broadcast parameters is imposed on all channels given.
If the broadcast wave scan is executed in the digital terrestrial broadcasting receiver shown in FIG. 3, then a scan time Tscan for executing a broadcast wave scan for one channel is expressed in the following equation (1) assuming that a tuning processing time of the tuner 11b is T11b, a demodulation processing time of the OFDM demodulation section 12 is T12, a decode processing time of the decoder 13 is T13, and a control processing time for performing control from the CPU 14 to each block is T14:Tscan=(T11b+T12+T13+T14).  (1)
Further, since the demodulation processing time T12 of the OFDM demodulation section 12 can be brought into detailed form as a synchronous establishment processing time T12a of the synchronous establishment unit 12a, a conversion processing time T12b of the demodulation unit 12b, and an error correction processing time T12c of the error correction unit 12c, the following equation (2) is obtained from the equation (1):Tscan=(T11b+T12a+T12b+T12c+T13+T14)  (2)
Assuming now that the number of all given channels is N and a total time necessary for the entire broadcast wave scan is Tt, the following equation (3) is obtained:
                                                        Tt              =                              Tscan                ×                N                                                                                        =                                                (                                                                                                                                          T                            ⁢                                                                                                                  ⁢                            11                            ⁢                            b                                                    +                                                      T                            ⁢                                                                                                                  ⁢                            12                            ⁢                            a                                                    +                                                      T                            ⁢                                                                                                                  ⁢                            12                            ⁢                                                                                                                  ⁢                            b                                                    +                                                                                                                                                                                          T                            ⁢                                                                                                                  ⁢                            12                            ⁢                                                                                                                  ⁢                            c                                                    +                                                      T                            ⁢                                                                                                                  ⁢                            13                                                    +                                                      T                            ⁢                                                                                                                  ⁢                            14                                                                                                                                )                                ×                N                                                                        (        3        )            
However, the digital terrestrial broadcasting receiver shown in FIG. 3 involves the following problems.
In the digital television broadcasting of the present situation, as shown in FIG. 2, the situation is that the number of all given channels is 50 and the number of channels at which broadcasting is being conducted in its region is less than 10. Since it is necessary to properly recognize that broadcasting is not conducted over channels greater than or equal to the remaining 40 channels, there is a need to sufficiently take the scan time Tscan at which the presence or absence of broadcast can be determined. The scan time Tscan for performing retrieval and determination needs a longer period of time with respect to each channel including a broadcast parameter. Therefore, the total time Tt for retrieving all the given channels needs an immense amount of time. Thus, it has become an important subject or challenge for a portable terminal which frequently changes in broadcast area.